The present invention relates to a sensor apparatus and a corresponding method for generating an output signal of a sensor apparatus.
WO 99/42789 discloses an apparatus for detecting a passage between a point on a body and a reference position. The apparatus comprises a pair of magnetic field sensors, each of the sensors generating an output signal based on the size of the magnetic field which passes through the sensor. A difference-forming circuit is provided which receives the output signals of the sensors and generates a difference signal with a peak value when the point of the body is positioned between the pair of sensors. Furthermore, a peak value detector is provided which detects the peak value in the difference signal. Moreover, the apparatus comprises a threshold value circuit which receives the difference signal, for the purpose of generating a gate signal if the magnitude of the difference signal exceeds a threshold level, the gate signal enabling the peak value pulse to be forwarded to an output terminal of the apparatus, and preventing peak value pulses from being forwarded to the output terminal of the apparatus in the absence of the gate signal. Finally, the apparatus comprises a threshold value setting circuit for setting the threshold value in accordance with the magnitude of the difference signal.
DE 197 01 262 A1 discloses a detector for passing magnetic articles with automatic gain control.
EP 0 642 029 A1 and EP 0 642 029 B1 disclose a rise-activated Hall voltage sensor. This sensor comprises a circuit branching device which is arranged in such a way that it follows a positive rise, the subsequent positive peak value of the Hall voltage is held at the detector output and the Hall voltage is applied to a comparator, so that, if, after incipient impinging [sic] of the Hall voltage from the positive maximum value, the increasing difference between the Hall voltage and the held voltage exceeds a predetermined magnitude, the resulting comparator output pulse indicates the beginning of a variation in the strength of the surrounding magnetic field at the Hall element.
Although applicable, in principle, to a wide variety of sensor apparatuses, the present invention and the problem area on which it is based are described with reference to a motor vehicle crankshaft sensor.
As is known, sensors are used to detect the movement or the positional state of rotating parts. Examples thereof are crankshaft, cam shaft, transmission and ABS sensors in motor vehicles. The sensors used are preferably Hall sensors which sense the variations in a magnetic field. For this purpose, by way of example, a permanent magnet is fitted to a stationary part in order to generate a magnetic field. This magnetic field is then modulated, depending on position, by a gearwheel fixed to the rotating part or by another ferromagnetic transmitter. In this case, the Hall sensor is preferably situated between the permanent magnet and the gearwheel or transmitter and can thus detect fluctuations in the magnetic field. By way of example, if a tooth of the gearwheel lies in the magnetic field, then a xe2x80x9chighxe2x80x9d output signal is supplied, while a gap between the teeth brings about a xe2x80x9clowxe2x80x9d output signal. In this way, the instantaneous position or annular velocity of a rotating part can be inferred from the signal output by the Hall sensor.
The signal supplied by such a sensor is significantly influenced by the operating conditions under which the sensor is used. These operating conditions include unavoidable imponderables such as, for example, operating temperature or size of the air gap, etc. Despite the fluctuations caused by the operating conditions, the sensor should supply an output signal that is defined as well as possible. In other words, the output signal should have a well-defined profile independently of the fluctuations caused by the operating conditions. The reason for this is as follows: if a sensor apparatus supplies a sinusoidal signal, for example, then a well-defined behavior of a system disturbed by the sensor apparatus can be obtained when switching operations in the system which depend on the output signal of the sensor are performed at the zero crossings of the signal. This is because these zero crossings are independent of the respective signal amplitude and, moreover, have a large edge steepness.
It goes without saying that a switching point other than zero crossing or signal center may possibly also be advantageous in the case of other signal waveforms of the output signal of the sensor.
During the evaluation of the output signal of a sensor for switching a system controlled by said sensor, a switching point should thus be complied with independently of the signal amplitude of the output signal of the sensor, which applies even to those low signals. In detail, VDI Reports 1287, 1996, pages 583 to 611, xe2x80x9cA new generation of Hall-effect gearwheel sensors: advantages through the combination of BIMOS technology and new packaging formulationsxe2x80x9d, describes a sensor arrangement in which firstly the amplitude of the output signal of a sensor is normalized if appropriate with the aid of an analog-to-digital converter. The signal peak values are detected with the aid of two further analog-to-digital converters and with digital-to-analog converters. A switching threshold is derived and defined from said peak values. In this way, finally, it is possible to achieve a system behavior that is essentially independent of temperature fluctuations and the width of the air gap. The outlay required for this sensor arrangement is relatively high, however, since gain matching and numerous analog-to-digital converters are required.
A circuit arrangement for calibrating switching points of a decision unit driven by an analog input signal, independently of a DC component contained in the input signal besides an AC component, is known, the input signal having upper and lower signal peaks which are in a selectable fixed ratio to one another. Provision is made, in particular, of peak detectors for determining the upper and lower signal peaks of the input signal; a controllable reference unit for providing a reference signal; a computation unit for determining the average value; a comparison unit; a regulating unit for compensating for the DC component of the input signal and a second regulating unit for oppositely tracking the reference value, said second regulating unit being connected downstream of the comparison unit on the input side and being connected to the reference unit on the output side.
In particular, the output of the sensor apparatus is blocked during the calibration phase. In the case of sensor apparatuses for movement and position identification, however, it is often important to correctly identify even small movements or the beginning of a movement. In the case of sensors which do not operate in a static manner, but rather operate by means of filtering or self-calibration in order to obtain a higher accuracy, the problem can arise, therefore, that, during the transient recovery times, the system operates either only inaccurately or, alternatively, not at all, so that the initial information is lost. A known sensor apparatus needs, for example, a time of six zero crossings until it supplies correct output information after the conclusion of the calibration phase. The time through to that point is required in order to set the internal circuit parameters in such a way that the circuit has suitable operating points.
Static sensors without adaptation do not have this problem. By the same token, they usually also have reduced sensitivity, which restricts the range of use. Sensors with a filter can react to the first or second zero crossing, but usually require a relatively long time before the parameters have adapted to the current operation to an extent such that the specified accuracy is also achieved.
Therefore, it is an object of the present invention to provide a sensor apparatus and a method for generating an output signal of a sensor apparatus in which relatively reliable output information can also be obtained during the calibration phase.
The sensor apparatus according to the invention has the advantage over the known solution approaches that there is no system dead time during the calibration phase.
The idea on which the present invention is based consists in the fact that, during the calibration phase, that is to say while e.g. the circuit searches for minima and maxima for defining the offset, the output information is not fundamentally blocked or ignored, rather precisely these minima and maxima are utilized as additional information sources.
More precisely, it is assumed that minima and maxima are sought in a signal profile of such a sensor apparatus, while the desire is to output an item of switching information at the signal zero crossings, for example a switching to H for minima and a switching to L for maxima. If a maximum has been found, then it can be assumed that a negative zero crossing will now soon occur, while in the event of a minimum being identified, it can be assumed that a positive zero crossing will soon occur. The temporal offset between the identification of the extrema and the actual zero crossings is by its nature unknown in this case, but it is possible in this way to generate a signal which indicates exactly the same number of zero crossings as are actually contained in the original signal.
Thus, the sensor apparatus based on this principle does not operate particularly accurately with regard to phase angle, since a maximum or a minimum is identified as such only if the actual signal already deviates again considerably from the maximum value or the minimum value. However, neither an item of information too many nor an item of information too few is generated with regard to the fictitious zero crossing, and therefore a distance covered per unit time is reproduced correctly, namely by the time interval between two adjacent fictitious zero crossings that are determined in this way.
In accordance with one preferred development, the zero crossing establishing device is configured in such a way that it establishes a fictitious zero crossing when the magnitude of the amplitude of the sensor signal has fallen by a predetermined proportion after an extremum defined in a phase-shifted manner.
In accordance with a further preferred development, the extremum defining device determines the minima of the analog sensor signal as follows: successive storage of a respective smallest value of the analog sensor signal until the difference in magnitude between a present larger signal value and the smallest signal value stored last is greater than a predetermined threshold. If the difference in magnitude between a present larger signal value and the smallest signal value stored last is greater than a predetermined threshold, definition of the smallest signal value stored last as minimum.
In accordance with a further preferred development, the extremum defining device determines the maxima of the analog signal as follows: successive storage of a respective largest value of the analog sensor signal until the difference in magnitude between a present smaller signal value and the largest signal value stored last is greater than a predetermined threshold. If the difference in magnitude between a present smaller signal value and the largest signal value stored last is greater than the predetermined threshold, definition of the largest signal value stored last as maximum.
In accordance with a further preferred development, the second output signal generating device is configured in such a way that it determines a rotational speed from the fictitious zero crossings established.
In accordance with a further preferred development, the analog sensor signal has an AC voltage component and a DC voltage component. The first output signal generating device has a calibration device for determining the DC voltage component of the analog sensor signal from the difference between successive minima and maxima during the calibration phase.
In accordance with a further preferred development, the first output signal generating device has a subtraction device for subtracting the DC voltage component determined from the analog sensor signal for the purpose of forming a corrected analog sensor signal; and a comparator device for comparing the corrected analog sensor signal with a reference signal and supplying a corresponding first output signal.
An exemplary embodiment of the present invention is illustrated in the drawings and explained in more detail in the description below.